vim-patch:8.2.3184: cannot add a digraph with a leading space

Problem:    Cannot add a digraph with a leading space.  It is not easy to list
            existing digraphs.
Solution:   Add setdigraph(), setdigraphlist(), getdigraph() and
            getdigraphlist(). (closes vim/vim#8580)
6106504e9e

Use GA_APPEND_VIA_PTR in registerdigraph().
Use tv_list_append_*() in getdigraphlist_appendpair().
Put the error messages in digraph.c.
E196 is N/A.
Remove mentions about 'encoding' being non-Unicode.
Nvim doesn't support setting encoding=japan, so skip a test.

src/nvim/digraph.h

@@ -2,6 +2,7 @@
 #define NVIM_DIGRAPH_H
 
 #include "nvim/ex_cmds_defs.h"
+#include "nvim/eval/funcs.h"
 #include "nvim/types.h"
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS